Collision-induced absorption (CIA) from molecular hydrogen is a dominant
opacity source in the atmosphere of cool white dwarfs. It results in a
significant flux depletion in the near-IR and IR parts of their spectra.
Because of the extreme conditions of helium-rich atmospheres (where the density
can be as high as a few g/cm3), this opacity source is expected to undergo
strong pressure distortion and the currently used opacities have not been
validated at such extreme conditions. To check the distortion of the CIA
opacity we applied state-of-the-art ab initio methods of computational quantum
chemistry to simulate the CIA opacity at high densities. The results show that
the CIA profiles are significantly distorted above densities of 0.1g/cm3 in a way that is not captured by the existing models. The
roto-translational band is enhanced and shifted to higher frequencies as an
effect of the decrease of the interatomic separation of the H2​ molecule. The
vibrational band is blueward shifted and split into QR​ and QP​ branches,
separated by a pronounced interference dip. Its intensity is also substantially
reduced. The distortions result in a shift of the maximum of the absorption
from 2.3μm to 3−7μm, which could potentially explain the
spectra of some very cool, helium-rich white dwarfs.Comment: 12 pages, 13 figures. Accepted for publication in The Astrophysical
Journa